The present invention relates to an injection-molding machine of the hot runner type and, more particularly, to a bimetallic clamp arrangement for securing a heater element in a desired position around a nozzle body for feeding liquified materials such as plastic, metal, etc. to an injection gate of a mold cavity.
To maintain the fluidity of the oncoming plastic material during closure of the injection gate, enabling its use in a following molding cycle, it is desirable to form a terminal part of the sprue channel as an axial bore of a nozzle of good thermal conductivity closely surrounded by an electrical heating element. Some nozzle manufacturers go through costly processes to integrate heaters into the nozzle body. The primary disadvantage with this approach however is that should the heater get damaged the nozzle is lost as well. Thus, it is preferred that the heating element be removably mounted onto the nozzle body for ready replacement. One of the primary challenges associated with this technique is holding the heating element firmly against the external surface of the hot runner nozzle housing so as to maximize heater performance and provide precise temperature control.
A variety of different approaches have been used to clamp heating elements to a nozzle body. One approach includes a clamping arrangement that mechanically presses the heater onto the nozzle body. An example of one such mechanical clamping arrangement is illustrated in U.S. Pat. No. 4,268,241 to Reese et al. As shown therein, an annular heater is held in position against a shoulder or tapering surface portion of the nozzle body by a nut threaded onto a gate-side extremity of the nozzle body.
Still other mechanical press type approaches include the heater band constructions shown on page 24 of the Husky Hot Runner Manual published by Husky Injection Molding Systems, Ltd. and in an advertisement for an axial clamp band heater published in the June 1992 edition of Modern Plastics, page 136. The axial clamp band heater shown in the advertisement differs from the more traditional heater band construction shown in the manual in that the clamping screw is aligned with the longitudinal axis of the heater and is not perpendicular to it as in the conventional construction. The axial clamp band heater has the advantage over the conventional construction of taking up less space since a large clearance is not needed for access to the screw.
The principal disadvantages of these mechanical press type approaches have been the cumbersome nature of the clamp assembly and the need to machine a pocket for the clamp screw. Still another disadvantage seen with these approaches is the uneven axial temperature distribution of the heaters. Typically, the heater coil has no profiled wattage. The heat demand of the hot runner nozzle is at the tip and the locator is different as it is at the center of the shaft. To achieve isothermal conditions and to improve the nozzle temperature profile, heat should be generated only where it is needed.
Still another approach for securing a heater to a nozzle body is exemplified by the D-M-E Gate-Mate Hot Sprue Bushing. This approach uses an external heater wrapped around the nozzle housing without any apparent clamping means. It is believed that this device stays in place by friction. By manually uncoiling the heater when it is slid over the housing, its internal diameter increases sufficiently to allow it to slide on. When released the coiling action of the heater causes a gripping of the housing or nozzle body. The primary disadvantages to this approach are that the heater is easily damaged and the absence of a firm grip which allows the heater to move or slip.
Accordingly, it is an object of the present invention to provide an improved band heater-clamp arrangement for securing an external heater to a nozzle body. It is a further object of the present invention to provide a band heater-clamp arrangement as above which has a compact design and a simple construction.
It is still a further object of the present invention to provide a band heater-clamp arrangement as above that is easy to install and to maintain.
It is yet a further object of the present invention to provide a band heater-clamp arrangement as above which has improved reliability and provides an improved heat profile.
Still other objects and advantages to the band heater-clamp arrangement of the present invention will become more apparent from the following description and drawings wherein like reference numerals depict like elements.